Tumor cells or cancer cells that circulate within the blood of a cancer patient are called CTCs. CTCs are thought to be a cause of metastatic cancers, and it has been suggested that CTC detection may be effective in early diagnosis of metastatic cancers. However, CTCs are one type of rare cell, and there are extremely few CTCs within blood, and there is, for example, approximately one CTC per from 108 to 109 blood cell components. Therefore, several methods of isolating CTCs have been proposed. For example, a method using a microfluidic device that binds a capture antibody to magnetic particles, that have immobilized anti-EpCAM (epithelial cell adhesion molecule) antibody, or to the resin surfaces of columnar structures or the like, a method of isolating by using a filter and by utilizing the difference in sizes of CTCs and blood cells, and the like are known as isolating methods (Isolation of rare periphery circulating tumor cells in cancer patients by microchip technology. Sunitha Nagrath et al. Nature, 2007, 450: 1235-1239), (Japanese Patent Application Laid-Open (JP-A) No. 2011-163830, JP-A No. 2013-42689, JP-A No. 2005-10177, Japanese Patent National Publication No. 2007-525642, JP-A No. 2009-106936).
A microfilter in which the material of the filter is formed of a polymer such as parylene (US Patent Application Publication No. 2011/0111412 A1) or polycarbonate (F. Farace et al., Br. J. Cancer, 2011, 105: 847-853) or the like, or silicon, a metal (JP-A No. 2013-42689), or the like, and that has on the surface thereof numerous pores of from 7 to 10 μm, is used in CTC isolating methods that use a filter. In principle, the filter has a pore size that is such that blood cells are passed therethrough and CTCs are captured on the filter.
In the metal filter disclosed in JP-A No. 2013-42689, a preferred metal is selected from gold, silver, copper, aluminum, tungsten, nickel, chromium, stainless steel and alloys thereof, and further, preferably, the length of the short side of the through-hole is from 5.0 to 15.0 μm, and the average opening ratio is from 0.1 to 50%, and the thickness of the filter is from 3 to 100 μm. In the Examples thereof, a nickel filter (average opening ratio: 1.4%, size of pore: 8×30 μm, shape of pore: rounded rectangular) is used and recovers human small cell lung cancer cell lines that have spiked within the blood. In the experiments thereof, it is stated that the cancer cell recovery rate is 74.9±10.5%, and the residual white blood cell count is 697±84.